(a) Field of the Invention
The present invention relates to epoxy resin molding materials for sealing of electronic components, particularly, to epoxy resin molding materials which are excellent in thermal shock resistance, thermal stability and moldability and are particularly suitable for the plastic packaging of large scale integrated circuits (LSI) for surface mounting.
(b) Description of the Related Art
Epoxy resin molding materials have been mainly and widely used for the sealing of electronic components, such as transistors and integrated circuits (IC). The reason for it is that epoxy resins have well-balanced properties including the electrical properties, thermal stability, mechanical strength and adhesive properties to inserts. In particular, the combination of o-cresol novolak epoxy resins and phenol novolak hardeners brings about especially well-balanced properties, and the use of base resins of such a combination has become the mainstream in the preparation of molding materials for sealing of integrated circuits.
However, as the packages of integrated circuits are given as typical examples, packages of electronic components are being miniaturized and becoming thinner, so that cracking of packages caused by cyclic cooling and heating has become a problem. A known means of improving the thermal shock resistance of epoxy resin molding materials is the modification of epoxy resins with silicone polymers. Silicone polymers in general are immiscible in epoxy resins and disperse in a form of particulates, thereby improving the thermal shock resistance of the molding materials. Although liquid silicone polymers improve the thermal shock resistance of molding materials because of their immiscibility, they have a shortcoming of bleeding during molding, which causes flash (bleeding from the clearances in molds) and deteriorates of the appearance of molded products.
When siloxane compounds reacted previously with epoxy resins or hardeners are used in molding materials in order to prevent the bleeding during molding, usual decrease in glass transition temperature becomes a problem.
In order to solve these problems accompanying the bleeding of the liquid silicone polymers, there have been proposed some methods, for example, a method wherein self-curing liquid silicone polymers are dispersed in base resins and are then allowed to self-cure, and the solidified resin mixtures obtained thus are mixed with other ingredients (Japanese Patent Application Kokai Koho (Laid-open) No. 63-20324 and Japanese Patent Application Kokai Koho (Laid-open) No. 63-241021). Nevertheless, such methods cannot be a satisfactory measures since self-curing liquid silicone polymers contain low molecular weight volatile matters, which causes the same problems as those caused by the modification with liquid silicone polymers.
As for the thinned integrated circuit packages for surface mounting, they are different from the conventional packages for pin-insertion mounting in the method of mounting. The conventional packages for pin-insertion mounting are not exposed directly to high soldering temperatures since soldering of the pins are carried out from the reverse of a wiring board after the pins are inserted in the lead mounting holes of the wiring board. On the other hand, since the packages for surface mounting are temporarily secured on the surface of a wiring board and are then connected by dip soldering, reflow soldering or the like on the same surface of the wiring board, they are directly exposed to the soldering temperature. When the integrated circuit packages have absorbed moisture, rapid expansion of the hygroscopic moisture occurs during soldering, resulting in the cracking of the packages. This phenomenon has become to a serious problem accompanying to surface-mounted integrated circuits.
This phenomenon is also due to flexibilizers which are additives for reducing the stress generated in the interface between large scale integrated circuit chips and sealing materials. The reason is that the conventional flexibilizers are siloxane compounds, which have high permeability of moisture and decrease the strength and the adhesive properties at high temperatures.
Since the problems described above are inevitable in the integrated circuit packages of the conventional base resin compositions, there are taken measures of shipping dampproofing-packed integrated circuits or drying sufficiently integrated circuits chips prior to mounting on wiring boards. These measures, however, require much labor and high cost.